(1) Field of the Invention
The present invention relates to an electrostatic chuck, and particularly to an electrostatic chuck for holding a wafer in a semiconductor-producing apparatus.
(2) Related Art Statement
At present, electrostatic chucks are used for attracting and holding semiconductor wafers in finely working, e.g., transferring, exposing, film-forming by CVD, washing, etching, and ding the semiconductor wafers.
As disclosed in, for example, JP-A 8-55,900, such an electrostatic chuck consists of a discoid positive electrode 2 and a discoid negative electrode 3 buried in a substrate 1 made of a ceramic material or the like as shown in FIGS. 1 and 2. An object 7 to be treated, such as a semiconductor wafer, is attracted onto a main plane 1A of the substrate 1 through an electric field generated by applying a given voltage between the electrodes from a DC power source 6. FIG. 2 is a plane view of the electrostatic chuck of FIG. 1 as viewed from an upper side. In FIG. 2, the object 7 is omitted for facilitating the explanation.
The prior art electrostatic chuck has a relatively large electrode gap 8 between the positive and negative electrodes 2 and 3, which results in an electric field that is insufficient to hold the object 7 to be treated near the gap 8 between the electrodes.
For the above reason, the object 7 does not fully contact the main plane 1A of the substrate 1 at the gap between the electrodes. Consequently, heat is not fully conducted to the object 7 from the substrate and the entire object 7 cannot be uniformly heated.
Further, since the attracting force may vary dependent upon any difference in a buried depth between the positive and negative electrodes 2 and 3 and different polarities of the applied voltage, the degree for the object 7 to contact the main plane 1A of the substrate 1 differs between a zone where the positive electrode 2 is positioned and a zone where the negative electrode 3 is positioned, which causes variations in the heat conductivity.
It is therefore an objective of the present invention to provide an electrostatic chuck which can uniformly attract an object to be treated and uniformly transfer heat to that object.
The present invention relates to an electrostatic chuck comprising a substrate having an inner electrode and an outer electrode made of a metal concentrically buried therein. A plurality of embossed portions are formed on a main plane of the substrate. A projecting portion is made of the same material as that of the plurality of embossed portions and is formed on substantially an entire outer peripheral portion on the main plane of the substrate on which said plurality of the embossed portions are formed. When positive and negative potentials are applied to the inner and outer electrode, respectively, or vice versa, an object to be treated is supported by the plurality of embossed portions and the projecting portion. A sum of a total area of upper surfaces of the embossed portions and the projecting portion in a zone of the substrate in which the outer electrode is located is in a range of 0.7 to 1.3 of a total area of upper surfaces of the embossed portions in a zone of the substrate in which the inner electrode is located.
These and other objects, features and advantages of the invention will be apparent from reading the following description of the invention and attached drawings, with the understanding that some modifications, variations and changes of the same could be easily made by the skilled person in the art to which the invention pertains.